Color developers produce a combined silver and dye image, and if clear colors are to be obtained the silver image must be removed. This can be accomplished by oxidizing the metallic silver with a suitable oxidizing agent, commonly referred to as a bleaching agent. This is followed by dissolving any remaining light-sensitive silver halide in a silver halide solvent, commonly referred to as a fixing agent. These bleaching and fixing processes may be performed sequentially in two separate solutions or in the alternative, the bleaching agent and fixing agents can be combined into a single bleaching-fixing solution.
The form in which the bleaching and fixing agents are typically employed is a dilute aqueous solution and thus it is not generally feasible, from an economic standpoint, to package, transport, and store solutions of working strength, since this would involve the packaging, transport and storage of large amounts of water. As such, the bleaching and fixing solutions are prepared immediately before use by dissolving the necessary ingredients in water. In the alternative a bleach/fix solution may be prepared by mixing bleaching and fixing solutions together that have been prepared separately such as disclosed in U.S. Pat. No. 5,070,004 issued to Yoshihiro Fujita et al. Preparation immediately before use is necessitated by the fact that individual components of the separate solutions can chemically react and quickly decompose and/or oxidize reactive components thereby diminishing the useful life of the working solution.
The prevalence of automatic processing systems used in small photographic service laboratories called "mini labs", that are supervised by technicians with limited technical background, necessitates the need for providing almost foolproof premixed solutions. However, as stated above, full working solutions that are ready-to-use make shipping more expensive, and further, the containers take up valuable storage space. To overcome these disadvantages, there has been a trend to reduce the size of packaging, and thus, provide a savings in shipping costs. One way to achieve this goal is to make either all-powder or liquid bleaching, fixing, and/or bleach-fixing concentrates. These concentrates merely have to be dissolved in or diluted with water to form a working solution.
However, because of the serious problem of unwanted chemical reactions that occur between the components, the bleaching and fixing concentrates are usually separated into two or more parts which are subsequently combined to form a working solution. For instance, a two-part bleach-fixing liquid concentrate is disclosed in U.S. Pat. No. 4,956,268 issued to Kiyoshi Nakazyo et al, wherein a first concentrate comprises a silver halide solvent and the second concentrate contains an oxidizing agent. The components are packaged separately from each other to prevent chemical reactions that form undesirable reaction products, such as crystallized precipitates. For instance, when a bleaching concentrate containing a ferric ion solution is put together with a fixing concentrate containing a thiosulfate solution the concentrate will have a very short shelf life because the ferric ion can be reduced to a ferrous ion and the thiosulfate can sulphurize thereby rendering the combined concentrate inactive.
While the higher degree of concentration will reduce some of the volume of water the two separate concentrates are still not sufficiently compact to substantially reduce the cost of shipping and/or the necessary storage space.
In an attempt to provide a one-part concentrate, U.S. Pat. No. 5,310,633 issued to Yoshida et al discloses a one-part powdery bleach-fixing composition. Powdered components tend to solve the disadvantages of shipping and storing separate liquid concentrates but lumping or caking of the dry components, especially the thiosulfate, can occur during shipping and/or storage thereby making it difficult to provide a homogenous solution upon dilution.
U.S. Pat. No. 3,751,251 provides for a one-part liquid bleach-fix concentrated regenerator. As described in this reference an aqueous solution of an aminopolycarboxylic acid and a thiosulfate ion are mixed along with a sufficient amount of a sulfite ion, in the form of gaseous sulfur dioxide, to adjust the pH of the solution to between 5.4 and 6.0. However, this product still has a limited shelf life due to undesirable crystal formation in the concentrate.
Accordingly, there is a need for improved one-part bleach-fix liquid concentrates which can be shipped economically, require minimal storage space, eliminate the need for mixing several different components, retain oxidizing ability and prevent crystals from forming in the concentrate.